Since plastic materials are light in weight, excellent in toughness and are easily dyeable, they have been used heavily for various kinds of optical materials, in particular, a lens for spectacles in recent years. Examples of performances required for optical materials, in particular, for an optical material for a spectacle lens include a low specific gravity as physical properties, a low degree of yellowness as chemical and thermal properties, high heat resistance and a high strength as mechanical properties, and examples of optical performances required include high transparency, a high refractive index and a high Abbe's number. Whereas the high refractive index enables the lens thickness to be thin-walled and the high Abbe's number reduces the lens chromatic aberration, the higher the refractive index becomes, the lower the Abbe's number becomes. Therefore, studies for improving the two together had been carried out. Among the studies, a method of using an episulfide compound shown in Patent Document 1 was provided as a major method. As another example, in addition, an optical material having the refractive index of 1.73 or higher by means of using an optical material comprising a polyepisulfide compound shown in Patent Documents 2 and 3 was proposed.
On the other hand, for a lens whose thickness has been reduced accompanied by upgrading the refractive index, a high strength is required more than ever before from the viewpoint of safety and the like. For example, in order to apply a frame, so-called a two-point frame, it is necessary to bore a hole in a lens with a drill and at this time, it is required to have enough strength not to generate cracks and/or breaks. Moreover, in order to withstand for the use of a two-point frame, the lens is required to have enough strength not to be broken even if the frame is bending. At the same time, since it is standard that the plastic lens is hard-coated for protecting its surface and heat is applied at the time of hard coating, heat resistance is also required.
Because of this background, studies for satisfying all of a high refractive index, a high strength and heat resistance at once had been carried out so far. Among the studies, a method of using a xylylenedithiol compound shown in Patent Document 4 was provided as a major method. According to this study, both of a high strength and high heat resistance were achieved for the material having the refractive index of 1.70, whereas, for the material having the refractive index of 1.73 or higher, any materials having both of a high strength and high heat resistance could not be obtained.
Therefore, regarding the lens having a refractive index of 1.73 or higher using an episulfide compound, it is expected to develop a lens highly excellent in strength and in heat resistance.